There is concern over the environmental impact of emissions from power plants and other fossil fuel combustion sources. For example, the exhaust gas of coal-fired power plants contains chemical pollutants such as nitrogen oxides (“NOx”) and sulfur oxides (“SOx”), as well as particulates, which are also known as “fly ash”. Selective catalytic reduction (SCR) and selective non-catalytic reduction (SNCR) are means for converting nitrogen oxides (NOx) into diatomic nitrogen, N2, and water, H2O. In SCR, a catalyst is used in combination with a gaseous reductant, which is added to a stream of flue or exhaust gas and is absorbed onto the catalyst. In SCNR, the reductant is injected into the flue gas in a furnace within an appropriate temperature window. Additionally, flue gas conditioning with a gaseous reductant can also enhance electrostatic precipitator performance for removing fly ash. In SCR, SNCR, and fly ash removal systems, the reductant is typically ammonia or urea.
For example, commercial SCR systems are typically found on large utility boilers, industrial boilers, and municipal solid waste boilers and have been shown to reduce NO by 70-95%. More recent applications include diesel engines, such as those found on large ships, diesel locomotives, gas turbines, and even automobiles.
The NOx reduction reaction takes place as the gases pass through the catalyst chamber. Before entering the catalyst chamber the ammonia, or other reductant, such as urea, is injected and mixed with the gases. The chemical equations for using either anhydrous or aqueous ammonia for a selective catalytic reduction process are:4NO+4NH3+O2→4N2+6H2O  (Equation 1)2NO2+4NH3+O2→3N2+6H2O  (Equation 2)NO+NO2+2NH3→2N2+3H2O  (Equation 3)The reaction for urea as a reductant instead of ammonia is:4NO+2(NH2)2CO+O2→4N2+4H2O+2CO2  (Equation 4)
The reaction has an optimal temperature range between 350° C. and 450° C., but can operate from 225° C. to 450° C. with longer residence times. The minimum effective temperature depends on the various fuels, gas constituents and catalyst geometry.
In SNCR systems, the absence of a catalyst increases the temperature for the reduction reaction. For example, the temperature window for efficient operation of an SNCR system is typically between 900° C. and 1,100° C. depending on the reagent and conditions of the SNCR operation.
Compared to urea, ammonia is more reactive, is more easily dispersed uniformly into the flue gas stream and is active over a broader temperature range, as well as being more efficient. Urea, as such, while also an effective reductant, forms unwanted byproducts, such as carbon monoxide (CO) and nitrous oxide (N2O), both of which are now under critical scrutiny by environmental authorities.
Commonly urea is thermally hydrolyzed to form ammonia for exhaust gas treatment applications. The hydrolysis of urea to form ammonia can be broken down into two distinct reactions. The first reaction is a mildly exothermic reaction, wherein heat is given off as urea hydrolyzes to form ammonium carbamate. The second reaction, in which the ammonium carbamate is converted to ammonia and carbon dioxide, is strongly endothermic, which overall dominates the thermodynamics of the conversion of urea to ammonia and carbon dioxide, i.e., the overall reaction is endothermic. Therefore, the hydrolysis of urea requires a substantial amount of heat and quickly stops when the supply of heat is withdrawn. For example, the liberation of ammonia commences at around 110° C. and becomes rapid at around 150° C. to 160° C., with or without catalytic assistance.H2O+(NH2)2CO→(NH2)CO2−NH4++NH3+heat  (Equation 5)(NH2)CO2−NH4++heat→2NH3+CO2  (Equation 6)Excess water promotes the hydrolysis reaction, the overall reaction for which is as follows:(x+1)H2O+(NH2)2CO+heat→2NH3+CO2+(x)H2O  (Equation 7)
However, under the reaction conditions necessary to affect useful throughput, the water quality is important. For example, in a conventional thermal hydrolysis of urea to ammonia for an SCR system, an aqueous solution of urea is atomized through a spray nozzle into a heated vaporization chamber. As such, the excess water is also vaporized during the hydrolysis of urea to ammonia, thereby leaving behind any non-volatile substances such as minerals. Minerals and other non-volatile substances can adhere to equipment surfaces, such as spray nozzles and the vaporization chamber walls, and build up over time, which may lead to blockage of the spray nozzle or reduced heat transfer efficiency to the vaporization chamber. Thus, the water used in thermal hydrolysis systems needs to be demineralized.
Further, the thermal hydrolysis of urea method is also sensitive to the quality of the urea. For example, formaldehyde present in urea can negatively affect the performance of an SCR system in a way similar to that of using demineralized water.
In view of the foregoing, the hydrolysis of urea requires an external heat source to initiate the reaction, even when coupled with combustion engines, and also is sensitive to the extent of demineralization of the water and the quality of urea used in the hydrolysis. Therefore, more efficient methods for generating ammonia for exhaust gas treatment applications are needed.